Originally Posted by Marc D
There's alway the possibility you have a spark plug that has popped loose but you're pretty safe to assume there is an exhaust leak. At 286K miles a cracked manifold would certainly be possible but it's usually associated with broken manifold bolts that hold it on. The only way to know for sure is a smoke test to pinpoint the leak.
How would I go about doing a smoke test? I've seen Youtube videos on using Sea Foam but I've never done this and don't want to blow my engine up.